For various kinds of furnace for heating or firing, a hot-gas blowing fan is in some cases used for circulating or agitating the gas in the furnace to homogenize the temperature in the furnace and improve the heating efficiency.
In a solid oxide fuel cell, the working temperature for power generation is from 700 to 1,000° C. Therefore, hydrogen and fuel gas such as natural gas or coal gas as a supply source of carbon oxide, which become fuel, are heated to from 700 to 1,000° C. before supplying them to the fuel electrode of the fuel cell.
In this case, it is reported to use a so-called steam reformation method wherein the hot fuel gas and steam are reacted to produce pure hydrogen according to the following formula, before supplying them to the fuel electrode. This method improves the reaction efficiency of the fuel gas and increases power generation efficiency in comparison with a method that the fuel gas is straightly fed to the fuel electrode for reaction.(Reaction formula of the steam reformation) CH4+2H2O→CO2+4H2 
In order to carry out the steam reformation of the fuel gas, it is necessary to humidify the fuel gas. However, when the humidification is carried out by using water for industrial use or domestic use, impurities contained in the water contaminate or corrode the main body of the fuel cell, so that a significant negative impact is on the performance and durability of the fuel cell main body. Further, an attempt to install an equipment for removing completely the impurities in the water supply line was questionable in view of the installation space and initial investment, and therefore, was not realistic.
In the solid oxide fuel cell, water is produced as a reaction product by hydrogen and oxygen at the side of the fuel electrode to which a fuel gas is supplied. Namely, at the same time that the fuel gas supplied toward the fuel electrode reacts at the fuel electrode, it is humidified with the produced water. The produced water does not contain impurities. Accordingly, if the humidified fuel gas can be circulated for reuse, the steam reformation of a fuel gas by the humidification becomes possible before the reaction, whereby the power generation efficiency can be increased.
Further, for the fuel gas heated to from 700 to 1,000° C. to be supplied to the fuel electrode, it is impossible that hydrogen and carbon oxide as reactive gases can be reacted entirely in only once contact with the fuel electrode. By circulating the fuel gas for reusing, the fuel gas can be used effectively and the sensible heat of the fuel can be reused, whereby the power generation efficiency can be increased in this point also.
From the reason described above, the technique of using a hot-gas blowing fan for a solid oxide fuel cell to circulate a hot fuel gas has been studied intensively.
On the other hand, there can be considered to provide a structure that the temperature of a hot fuel gas is decreased to about 100° C. by means of a heat exchanger, and when the temperature is not more than 100° C., the fuel gas is pressurized with an ordinarily available fan, and then the fuel gas is heated again to a working temperature of from 700 to 1,000° C. However, the idea of this structure was no realistic at all when the problems of heat loss, the cost of the heat exchanger and the installation space were considered.
When the hot-gas blowing fan is applied to the solid oxide fuel cell, the following conditions should be satisfied.
1) Since a hot fuel gas is combustible and is deadly to human being depending on a fuel cell used, the hot fuel gas must not be leaked outside the system. Namely, the shaft sealing device for the rotating shaft connecting the motor with the impeller comprising rotary vanes and a disk should be completely gas-tight.
2) Since there is a case that the fuel cell is used as a dispersed power source in out-country, and the fuel cell system itself should be simple, the utility should not be used other than a d.c. power source supplied from the fuel cell system itself. Further, the amount of electric power for the blowing fan should be not more than about 5% of the generated power.
3) The fuel cell is installed as a dispersed power source for usual houses and small-sized apartments. Accordingly, the blowing fan used should be compact.
4) The initial investment should be low. Specifically, it is desired to be not more than 3% of the sales price of the fuel cell system.
5) In order to prevent condensation of the hot fuel gas, the temperature of the part exposed to the hot fuel gas should be always at least the dew-point temperature.
6) In order to prevent the breakage or deformation of the impeller, there should not be any danger of collision of a foreign matter having a size by which the breakage or deformation of the impeller during the rotation may cause.
As described above, from the viewpoints of safeness and economy, it is important for the solid oxide fuel cell that the hot fuel gas should not be leaked outside the system.
As the shaft sealing method for the hot-gas blowing fan, which has conventionally been utilized, there is a method that a first shaft sealing device is inserted on the rotating shaft at a position between a bearing and a cooling portion provided between an insulating layer and the bearing, and a second shaft sealing device is inserted on the rotating shaft at a position between a second bearing disposed at a low-temperature side of the rotating shaft and a shaft coupling disposed at the end of the rotating shaft, this method being generally utilized. As the first and/or second shaft sealing device, a gland packing, an oil seal, an O-ring, a labyrinth, a mechanical seal or the like is used.
Among these shaft sealing devices, the gland packing, the oil seal and the O-ring are made of rubber or a synthetic resin. Accordingly, these elements are sensitive to gas quality and temperature, and therefore, the service life can not be expected beyond several years. Especially, the solid oxide fuel cell has strong reducing properties because the hot fuel gas as fuel contains hydrogen and carbon oxide. Accordingly, the sealing technique using rubber or a synthetic resin is less reliability.
For the labyrinth or the mechanical seal, a purge gas is used to always push it in order to prevent a process gas sealed inside from leaking outside. In this case, the mixing of the purge gas with the process gas is unavoidable. For the solid oxide fuel cell, it is very important that the process gas should be pure for the performance of the fuel cell, and the mixing of the purge gas is generally impermissible. In case of using a purge gas for the solid oxide fuel cell, an inert gas such as nitrogen, helium or the like being expensive can be considered. However, the cost of the utility would thereby increase, with the result of an increase of cost per unit power. Further, when the solid oxide fuel cell is used as a dispersed power source for a usual house and a small-sized apartment, there arise problems such as the space for the purge gas cylinder, safety control, re-supply and so on, such being unrealistic.
As described above, any shaft sealing device without using the utility other than the power source, being compact, simple and completely gas-tight has not actually been proposed.
A bearing is fitted to the rotating shaft to cantilever the impeller. The upper temperature limit for allowing use of the bearing for a long-term under good conditions would be about 100° C. in consideration of the restriction of the upper temperature limit of the lubricant such as grease used for lubricating the bearing. It is necessary to remove the flux of heat transferred from the impeller to the rotating shaft through the heat insulating layer disposed between the impeller and the bearing, whereby the bearing can be cooled to a predetermined temperature or lower. Here, the removal of heat means that the flux of heat is taken to discharge it.
In the hot-gas blowing fan conventionally used for removing heat, such a method is generally employed, wherein a water-cooling jacket is provided between the bearing and the impeller which is directly exposed to a hot gas, in coaxial with the rotating shaft and in contactless therewith while it is in direct contact with the outer ring of the bearing, a cooling water cooled to, for example, not more than 30° C. is supplied to the water-cooling jacket to maintain the surface temperature of the water-cooling jacket to, for example, not more than 50° C. whereby the rotating shaft is cooled by heat radiation, and the bearing is cooled by the heat transfer between the water-cooling jacket and the outer ring of the bearing. Further, such a method may be employed, wherein the lubricating oil cooled to, for example, not more than 30° C. is brought to direct contact with the rotating shaft and the bearing to remove the heat under lubrication.
However, the conventional cooling method required devices such as a pump for circulating water or a lubricating oil as a heat-removing medium, a cooling device for cooling the heat-removing medium and pipes for connecting these, with the result of bringing drawbacks of making the entire system complicated and hindering the reduction of the installation space. In particular, such were the major negative factors in introducing the solid oxide fuel cell as a dispersed power source for a usual house or a small-sized apartment.
In the solid oxide fuel cell using natural gas as the fuel, the dew-point of the fuel gas, i.e., the process gas is about 70° C. Accordingly, when the heat-removing medium such as water or a lubricating oil having a low temperature as above-mentioned was used, there caused super-cooling and there caused dew condensation on and near a cooling section such as the water-cooling jacket, so that there created a problem of causing the deterioration of the fuel cell main body due to corrosion derived from the condensation of moisture, and elution or scattering of a contaminant to thereby affect deadly the performance and durability of the fuel cell.
Further, since it was necessary to prevent the deterioration of the water or the lubricating oil as the heat-removing medium and to compensate the reduced amount thereof, a continuous, maintenance-free operation of, for example, 24 hours×365 days×3 years was considered to be difficult.
In addition, if the power source for a device such as a pump for circulating the heat-removing medium is stopped due to power stoppage, or the supply of the heat-removing medium is stopped due to a failure of the device itself, measures have to be taken so as to stop the heating of the hot gas by means of an electric control mechanism or the like. In such case, the shaft sealing device or the bearing may suffer a fatal damage by the heat of the hot gas of from 700 to 1,000° C. inside the device or the heat from the heat insulating material heated to have a high temperature.
Further, a method in which the rotating shaft and the bearing are cooled directly with use of a cooling fan can be considered, as one way of thinking, although the sealing structure for the method is not realistic. In this case, the heat transfer coefficient indicating the ability of removing heat, of water is from 1,000 to 3,000 w/m2K while that of air is far smaller, i.e. from 10 to 30 w/m2K. Accordingly, if air is used to obtain the same cooling effect as that by water, the surface area of the heat-removing portion should be about from 100 to 300 times as much as the case of using water, and it was in fact difficult to provide the heat-removing section in a limited space around the rotating shaft and the bearing.
When a hot-gas blowing fan is used for the solid oxide fuel cell, it is necessary to drive a small-sized impeller at a high speed, rather than driving a large-sized impeller at a low speed, to satisfy specifications such as wind volume, wind pressure and so on in view of restrictions such as cost and the installation space. However, if a foreign matter hits the impeller rotated at a high speed, the impeller may be broken or deformed.
For example, when an impeller made of silicon carbide is used to drive it at an impeller's circumferential velocity of 205 m/sec, the allowable particle size of foreign matter which does not cause the breakage of the impeller even in collision, is not more than 1 mm according to experiments. It means that it is necessary to provide a dust collector not to suck a foreign matter having a particle size of more than 1 mm from the inlet port of the scroll under the above-mentioned condition.
Conventionally, the purpose of use of the hot-gas blowing fan was fairly limited; in particular, it was unnecessary to consider the incoming of a foreign matter. Accordingly, a dust collector for a hot-gas blowing fan requiring a low cost and a small installation space has not actually existed.
From these reasons, it was difficult to provide the technique capable of satisfying the conditions to apply a hot-gas blowing fan to the above-mentioned solid oxide fuel cell.
It is an object of the present invention to solve the problems of the conventional techniques and to provide a hot-gas blowing fan suitable for a solid oxide fuel cell.